Field of the Invention
The present invention relates to semiconductor wafers and more particularly to a unique multilayer epitaxial structure for use as a base material for semiconductor devices and to a method for fabricating same by epitaxial techniques which can be performed in a single continuous operation, in the chamber of a chemical vapor deposition reactor.
Semiconductors of many types are formed on wafers which are commonly made of silicon crystal material. The material is produced by an epitaxial growth process in accordance with well known chemical vapor deposition principles. The growth of the material takes place in vessels called chemical vapor deposition (CVD) reactors which have facility to control growth conditions such as temperature and pressure, as well as impurity concentrations.
Certain types of semiconductors such as power rectifiers, Zener diodes and multilayer devices such as transistors have specific electrical parameter requirements, for example, forward voltage drop, leakage current and breakdown voltages, which are difficult to accurately control by conventional diffusion means. The present invention is an all epitaxial process for creating base material for use in the fabrication semiconductor devices which provides accurate controls to permit formation of a structure which is capable of meeting such device electrical parameters requirements. In addition, the invention eliminates the need for an impurity diffusion.
The multilayer structure is epitaxially grown with a top layer having high impurity concentration and a middle layer having light impurity concentration. Those layers are deposited on a substrate of high impurity concentration.
The formation of heavily doped layers by epitaxial growth usually means slow growth rates. The presence of high impurity concentrations in the material leads to problems with "outdiffusion", where the high dopant concentration layers tend to contaminate low concentration layers, as the material containing the heavily doped layer is heated to high temperature for processing. Another problem associated with heavily doped epitaxially grown layers is "autodoping" in which dopant moves into the gas stream. This autodopant together with highly concentrated dopant gas used contaminates the reactor chamber and quartzware and therefore tends to degrade resistivity control on subsequent runs (the so called "memory effect"). Consequently, where high impurity concentrations are required, it has been heretofore necessary to form the doped layers by diffusion in a separate vessel called a diffusion tube.